Among other benefits, rapid solidification processing (RSP) allows the preparation of alloys in a desirable form of macroscopically homogeneous, fine-crystalline material in which micron or submicron crystal particles are contained in a matrix to provide improved properties. However, sufficiently rapid solidification often cannot be achieved when attempting to melt and quench complex multiphase alloys to produce submicron particles. This is the case especially when the solidification process takes place over too long a time. The particles to be formed in the matrix will then often grow to such a size that they are no longer useful in producing improved properties. This may occur because the temperature drops at too low a rate or because the solidification temperature interval is relatively large. Thus, the major reason for the inability to rapidly solidfy multiphase alloys coming from the phase diagram of the alloy is a relatively large difference in liquidus and solidus temperatures for the alloy. The problem in rapidly solidifying such alloys is that relatively large particles tend to grow because the temperature of the alloy must be dropped through the entire liquidus-solidus temperature interval prior to reaching the temperature where solidification is complete. Because in the prior art the cooling starts at the liquidus temperature of the final alloy, not only is the solidus-liquidus interval large, allowing large particles to form on cooling, but also the effective cooling rate at each temperature of the liquidus-solidus interval is reduced, again contributing to the growth of relatively large particles.
More specifically, the products of the solidification process in a multiphase alloy are dependent on the rates of nucleation and growth of the constituents. Through their effect on the relative rates of crystal nucleation and growth, two factors can be considered to control the number and size of dispersoid particles growing in the melt: First, the effective cooling rate at a given temperature and, second, the temperature interval between start and completion of the solidification process which, in the absence of appreciable supercooling, is close to the solidus-liquidus temperature interval. These two factors may be combined qualitatively by defining a cooling period during which the solidification process of the dispersoid phase proceeds. While, within limits, the cooling rate is controllable, the solidus-liquidus temperature interval is given for each alloy system, and may be unfavorable for microdispersion formation.
Since in the absence of supercooling, initial solidification takes place quite close to the liquidus temperature of an alloy, it is not possible to achieve an effective cooling rate of 10.sup.4 -10.sup.6 .degree.K./sec, typical of rapid solidification processing and a short cooling period in the case where the liquidus-solidus interval is large as compared to the liquidus temperature. For instance, with .DELTA.T=T.sub.L -T.sub.S, where T.sub.L is the liquidus temperature of the final alloy and T.sub.S is the solidus temperature, if .DELTA.T/T.sub.L is large, e.g. greater than 0.2, then neither a high effective cooling rate nor a short cooling period can be achieved. If a high effective cooling rate and a short cooling period cannot be achieved, then dispersoid constituents in the alloy grow above a submicron particle size as they grow and agglomerate during the relatively slow and lengthy cooling process. These large grown or agglomerated particles are thereafter relatively useless in providing improved properties, one of which is dispersion strengthening of the final alloy.
For example, it will be appreciated that dispersion strengthening requires a fine microdispersion of particles within a matrix in which the average diameters of the particles within the matrix are to be in the submicron range and desirably in the 10 nanometer of 0.1 micron range. Thus, with alloys having large liquidus-solidus temperature intervals, assuming the multiphase alloy is prepared ahead of time and then melted to form a single liquid solution, the resulting dispersoids in the alloy grow to such large size during rapid solidification that they do not form a submicron dispersion (microdispersion) in the final product.
By way of background, a method for the treatment of liquid metals described in U.S. Pat. No. 4,279,843 to N. Suh discusses a process in which two streams of liquid metals are made to impinge upon each other, with the result that eddys of one liquid form in the other. As described in this patent, if the temperatures are chosen properly, small particles of one stream of material will form and solidify in the stream of the other material which acts as a coolant. It will be appeciated that while this patent describes mixing of liquid alloy streams, there is no chemical reaction between any of the components.
As described by E. F. Caldin, "Fast Reactions and Solutions," Wiley, New York, 1964, a second prior art technique is the so-called "stop-flow" method which is extensively used in inorganic chemistry to measure fast reactions. In this process, two fluids are rapidly mixed by injection into a mixing tube and the ensuing solution reaction is monitored, usually by spectrometry. Here no reference is made to alloys having large liquidus-solidus intervals or any resultant product.
Reference is also made to U.S. Pat. No. 4,356,133 in which a reactant medium is transported across the surface of a body rotating at high speed and is discharged therefrom by centrifugal force. A chemical reaction is caused to occur in the medium after the discharge while the reactant medium is still connected to the surface of the body. While this patent indicates travelling chemical reactions, the patent is silent about the utilization of the process for rapidly solidified alloys, much less alloys having large liquidus-solidus intervals.
Further, U.S. Pat. No. 4,257,830 illustrates a number of different melt spinning processes including various nozzle configurations, there being no melt mixing to form a chemically reacted product.
Finally, U.S. Pat. No. 4,339,255 shows a method for forming a metallic dielectric or semiconductor modified amorphous glass material, e.g. a single phase material, which includes the steps of forming a fluid host matrix material on a substrate surface having relative movement thereto, such as a wheel, directing a fluid modifier material in a stream, as from a nozzle, towards a substrate surface in a direction such that it converges with the host matrix material while at the same time maintaining the temperature of the substrate or wheel between 4.2.degree. Kelvin and ambient room temperature while rotating the wheel at a velocity of 1,000 to 5,000 r.p.m. to obtain a surface velocity of between 1,000 to 4,000 centimeters per second for obtaining a rapid quenching of the host and modifier material as they contact one another at a rate of from 10.sup.4 to at least 10.sup.8 .degree.C. per second or more to produce a ribbon of modified amorphous glass material. It will be appreciated that in this patent the goal is the formation of a homogenous material, and not a heterogenous material for improved mechanical, magnetic, and/or electrical properties. What is produced in this patent is an amorphous material in the form of a homogeneous single phase material in which no second phase has begun to form. That is to say, no locally heterogenous material is formed, whether or not as a dispersoid. This patent does not envisage the final product having anything other than a homogenous structure, unlike precipitates which can produce improved mechanical, chemical and electrical properties. Moreover, any chemical reaction which could take place, would take place in free space which is extremely difficult to control and is therefore undesirable. Moreover, the advantage of melt mixing with respect to multiphase alloys having large liquidus-solidus temperature intervals is not discussed in this patent.